1. Field of the Invention
This invention relates to optical exposure and more particularly to Off-Axis Illumination (OAI) methods and apparatus for optical exposure tools.
2. Description of Related Art
U.S. Pat. No. 5,712,698 of Poschenrieder et al. for "Independently Controllable Shutters and Variable Area Apertures for Off Axis Illumination" describes use of an aperture plate including apertures that provide off axis illumination located between an exposure source and a photomask provided with variable area apertures by use of independently controllable mechanical shutters.
U.S. Pat. No. 5,453,814 of Aiyer for "Illumination Source and Method for Microlithographyl" shows an exposure system that using uses acousto-optical modulators, i.e. Bragg cells to change the frequency of light. The system separates a light source into a number of segments. Each segment is frequency shifted by a different amount. Each segment of light passes through a short focal length lens of a fly's eye array to be dispersed onto a mask plane for evenly illuminating a mask. The fly's eye array is comprised of Bragg cell units (of Acousto-optical modulators) that comprise a transparent crystal to which a radio frequency voltage is applied. The Bragg cells shift the frequency of the light passing therethrough.
U.S. Pat. No. 5,638,211 of Shiraishi for "Method and Apparatus for Increasing the Resolution Power of Projection Lithography Exposure System" discloses an illumination system for illuminating a mask. A condenser lens and an optical integrator element are followed by a spatial filters with windows arranged at the Fourier transform plane. Another spatial filter is located at the at the Fourier transform plane of the projection optical system. The spatial filters are formed by a light shielding plate or an electrooptic element such as a liquid crystal device or an electro chromic device.
U.S. Pat. No. 5,684,567 of Shiozawa for "Exposure Apparatus and Device Manufacturing Method for Projecting Light from a Secondary Light Source onto a Mask or Pattern" shows an exposure system having a series of optical integrators for shifting the frequency of light and also shows a CCD for monitoring the light quality.
U.S. Pat. No. 5,600,485 of Iwaki et al. for "Optical Pattern Recognition System Method of Ferroelectric Liquid Crystal Spatial Light Modulator" discusses SLM (Spatial Light Modulators) systems including a transmissive spatial light modulator 104 in FIG. 2 described at Col. 18, lines 34-51.
U.S. Pat. No. 5,701,185 of Reiss et al. for "Spatial Light Modulator Assembly for Adapting a Photographic Printer to Print Electronic Images" shows use of a SLM for an electronic printing process.
Pfauler et al. "High-Throughput Optical Direct Write Lithography", Solid State Technology (June 1997), pp. 175-176, 178, 180, 182 describes a direct write lithography system using a programmable phase-modulated SLM system in which the image is reflected from the SLM onto a semiconductor wafer. The spatial light modulator comprises an array of rectangular electrodes with a reflective, deformable viscoelastic layer on top. The SLM serves as a plane mirror in an optical system.
U.S. Pat. No. 4,846,694 of Erhardt for "Computer Controlled Overhead Projector Display" in which a computer controls a transmissive (transparent or semi-transparent) liquid crystal display overhead projector display.
U.S. Pat. No. 3,824,604 of Stein for "Alphanumeric Printing System Employing Liquid Crystal Matrix" shows a printing system using a LCD matrix.
U.S. Pat. No. 5,028,939 of Hornbeck for "Spatial Light Modulator System" shows a SLM.
U.S. Pat. No. 5,083,854 of Zampolin et al. for "Spatial Light Modulator with Improved Aperture Ratio" shows a SLM with improved aperture ratio.
U.S. Pat. No. 5,576,562 of Komuma for "Solid-State Imaging Device" shows an imaging device using photodetectors arranged in a matrix array and a CCD.
FIG. 1 shows a conventional prior art illumination system 12 wherein the Critical Dimension (CD) is close to the exposure wavelength, which reduces the image quality. A collimated beam CB of light is directed at a blade BL1 which contains an aperture AP through which a patterned beam passes down through condenser lens CL1 through mask M1 with window W1 therein and then the beam passes down through projection system PC onto the target workpiece which is a semiconductor wafer W. There is a dispersion of the beam from the -1 order beam to the 0 order beam to the +1 order beam with an angle of .THETA. between the -1 order beam and the +1 order beam. If the mask M1 has a line/space pitch of P, then for wavelength LAMBDA there are relationships defined by the equations as follows: ##EQU1##
FIG. 2 shows a prior art Off Axis Illumination (OAI) system 14 which is a modification of FIG. 1 in that the simple aperture blade BL1 has been replaced with an OAI blade BL2 which employs a conventional OAI mechanical blade. In FIG. 2, a collimated light beam CB is directed at a new OAI blade BL2 which contains at least apertures AP2 and AP3 through which a patterned portion of beam CB passes down through condenser lens CL1 through mask M1 with window W1 therein and then the beam passes down through projection system PC. There is a dispersion of the beam from the -1 order beam to the 0 order beam to the +1 order beam with an angle of .THETA. between the -1 order beam and the +1 order beam, but the -1 order beam is shown being directed away from the semiconductor wafer W. ##EQU2##
Therefore, the OAI is has improved resolution. Due to the incident angles the zero order and first order (+1) diffracted beams at the wafer W are equal. Therefore the optical paths are the same. The result is that there is no wavefront aberrations, so the Depth Of Focus (DOF) is improved significantly.
In the case of a conventional mechanical OAI blade, there is the problem that the mechanism creates instability and positioning errors for every insertion.
Another problem with a mechanical blade with the OAI arrangement of FIG. 2 is that it can include a limited number of patterns for the OAI blade and this makes it difficult to provide different OAI patterns for use. This is manifested by the use of rotatable shutters in Poschenrieder et al. U.S. Pat. No. 5,453,814, cited above.
FIGS. 3A-3D shows four different patterns of a conventional OAI blade which are mechanically inserted into an OAI aperture position.
In FIG. 3A, a blade is shown with a pattern SA (Small Annular) transmissive region TR1 and inner and outer opaque regions OP. Light passes through transmissive region TR0 but, on the contrary, light is absorbed or blocked by opaque regions OP.
In FIG. 3B, another blade is shown with a large annular LA pattern with a transmissive region TR2 and inner and outer opaque regions OP.
In FIG. 3C, a third blade is shown with quadrupoles with a set of transmissive regions TR3 and overall opaque region OP.
In FIG. 3D, a fourth blade is shown with quadrupoles with a larger set of transmissive regions TR4 and overall opaque region OP.